The present invention relates to a process for producing a plastic composition which can be processed into a fire-resistant material, more particularly, a composition containing ceramic fibers, organic bonding agent, water and clay.
The present invention also relates to the composition produced by this process, and the use of the composition in applications where fire-resistance is desired.
The art is aware of heat-insulating, ceramic fiber bodies, made of fire-resistant or fire-proof fibers and organic or inorganic bonding agents, having low stability and high compressibility, or having increased stability, density and nondeformability. For example, German Patent Specification DE-AS No. 12 74 490 describes a combustion chamber for furnaces, constructed by shaping a composition containing fibers and bonding agents, in such a manner that the concentration of bonding agent varies across the cross-section of the furnace wall. This reference mentions different types of clay, alkali-silicates, aluminum phosphates and colloidal siliceous earth, in an amount of 5 to 35% (optimum of 10%) by weight, as suitable bonding agents. However, such fiber bodies are not sufficiently suited for application of high stress, since one wall surface is dense and hard, and the other, opposite, wall surface is soft and flexible.
Another German Patent Specification, DE-AS No. 27 32 387, discloses a process wherein a mineral fiber-board, which has been hardened in advance with an organic synthetic bonding agent, is soaked in an aqueous solution of a bonding clay and subsequently hardened through tempering.
Furthermore, DE-AS No. 26 18 813 discloses fiber spraying-compositions which contain, in addition to a large amount of inorganic fibers, a small amount of bonding agent and/or inorganic components as well as an additional chemical bonding agent. These fiber spraying-compositions also contain 5 to 20% by weight of an oil, to avoid dust formation. Instructions for use of these fiber spraying-compositions expressly indicate that the inorganic fibers, such as rock wool, have to be used in a loosened condition.
It is also known that, to increase the density of the ceramic fibers, clay or other finely dispersed refractory materials can be added to the fibers, along with the usual bonding agents and the required quantity of water, in order to obtain compositions which, after processing into fire-resistant or fire-proof materials, have a higher density than the compositions containing only the ceramic fibers, bonding agent and water.
The art is also aware of the fact that products containing ceramic fibers can be manufactured by a vacuum suction process, as boards and moldings having densities of about 0.2 g/ml. For this purpose, the ceramic fibers and bonding agent, such as starch, colloidal silicic acid or a phosphate such as mono-aluminum phosphate or sodium polyphosphate, are sucked onto a screen surface in the form of a suspension containing about 1 to 5% by weight of the fibers. To achieve higher densities, and thus give special properties to the moldings manufactured by the vacuum suction process, other fillers can also be used, such as fire clay, alumina, silicon carbide or clays such as bentonite. However, there is a disadvantage in manufacturing moldings by the vacuum suction process, in that the bonding agent is present in the form of an aqueous solution or suspension, and the greater part of the bonding agent is removed with the water when the water is evacuated, meaning that if soluble phosphates are used, problems of waste water disposal are created. Additionally, the presence of other refractory fillers means that the density of the product decreases with increasing distance from the suction or screen surface used in molding, so that, especially with products having a large wall thickness, there are severe fluctuations in density throughout the cross-section of the wall.
It has been shown that the strength properties of a product, such as its compression strength, tensile strength, hot bending strength and resistance to abrasion, are improved with increasing density of the product. With vacuum-molded products, and also with products manufactured by previously known mixing methods, an increase in the density can only be achieved by adding larger amounts of fillers, such as standard clay or fire clay. However, this means that a molding composition containing, for example, 50% by weight of ceramic fibers and 50% by weight of fillers, will have a higher heat conductivity than a material containing a higher proportion of insulating ceramic fibers, so that the advantageous properties of the molding composition are not realized.